If some twisted genius vaporized all 10 trillion cells in your body -- along with the hair, the fingernails, and other tissue they create -- it would not leave empty space behind. A body-shaped cloud made of bacteria, viruses, and other former stowaways would hover briefly in the air. The cloud would outline your skin, delineate your lungs, trace your digestive tract. You might be gone for good, but your shadow biosphere would remain.

We got our first glimpse of these tiny tenants -- now known collectively as the microbiome -- in the late 17th century, when a Dutch lens grinder named Anton van Leeuwenhoek noticed a layer of white scum between his teeth. He mixed some of the gunk with pure rainwater and then placed it under one of his handmade microscopes. “I found, to my great surprise,” he wrote, “that it contained many small animalcules, the motions of which were very pleasing to behold.”

With the advent of fast DNA sequencing, today’s microbiologists can delve deep into this weird inner universe, and they’re just as amazed as Van Leeuwenhoek was. It’s not just the sheer quantity of microbial cells (100 trillion or so for one person alone) but also their diversity: Each of us is home to thousands of species of microbes, and no two people have quite the same mix.

We’re just beginning to learn the effects our microbiome has on us, but it’s clear that they can be profound. Certain species help digest food and synthesize vitamins; others guide the immune system. Medical researchers have linked obesity, heart disease, and anxiety to properties of the microbiome. In many cases, it’s not the individual species that seem to matter but the richness of the ecosystem. Just as the health of a forest depends upon diversity, our own health appears to benefit from the presence of a wide range of uninvited guests, many of which coevolved with us.